Acquired immunodeficiency syndrome ("AIDS") was recognized as early as 1979. The number of cases reported to the Centers for Disease Control (CDC) has increased dramatically each year since then, and in 1982 the CDC declared AIDS a new epidemic. Infection with the AIDS virus is highly variable. Initially, the virus replicates abundantly, with virus present in the central nervous system and the cells of the immune system. This is frequently accompanied by fevers, rashes, flu-like symptoms and neurological complaints. These symptoms generally disappear within a few weeks, as the amount of virus in the circulation drops. However, virus is still present in the immune cells, the cells of the nervous system, cells of the intestine, and in bone marrow cells. The victim typically dies two to ten years after the initial asymptomatic period, following a protracted and painful illness.
Human immunodeficiency virus (HIV), a retrovirus, has been determined to be the etiological agent of AIDS, as well as of a variety of related disorders, such as AIDS Related Complex (ARC) (see, e.g., Barre-Sinoussi, F., et al., Science 220, 868-871 (1983); Gallo, R. C., et al., Science 224, 500-503 (1984).
HIV infection begins when a virion or virus-infected cell binds to susceptible cells and fuses with them, injecting the core protein and viral RNA into the cell. The RNA is transcribed to viral DNA. The double stranded DNA migrates to the nucleus and is suspected to integrate into the cell's DNA. The viral DNA can remain dormant for an indefinite period of time, or the genes can replicate and be translated into viral proteins. The viral proteins are assembled into new virions that bud from the cell, spreading the disease.
HIV preferentially infects the T4 lymphocytes, immune cells that are important in helping to suppress infection in the body. As T-cells are destroyed by HIV, the body's immune system is impaired. One of the most serious complications of Acquired Immune Deficiency Syndrome is the proliferation of opportunistic infections that occur after a severe decline in immune function. "Opportunistic infection defined AIDS" is one of the final stages of the disease, typically characterized by a T4 cell count of 100 or less. Most patients die within two years of reaching this stage. The opportunistic diseases that occur most often at this stage are probably prevalent because the agents that cause them are ubiquitous in all humans, including healthy humans.
Opportunistic infections common among AIDS patients include various bacterial diseases caused by agents such as mycobacterium species (M. avium intracellulare and M. tuberculosis), Legionella sp., Salmonella sp., Shigella sp., and other bacteria that cause infections in which T-cells are important in host defense. Recently, mycoplasma bacteria have been isolated from lung biopsy tissue of HIV infected individuals. Opportunistic parasitic infections include Pneumocystis carinii pneumonia (PCP), toxoplasmosis (which infects the brain and leads to seizures and coma), chronic cryptococcosis (that can cause meningitis), and histoplasmosis (a frequent cause of chronic fever). An often observed secondary viral infection is cytomegalovirus, which is a cause of pneumonia, encephalitis, blindness, and inflammation of the gastrointestinal tract. This viral infection is typically a reactivation of a childhood infection that was well controlled before HIV infection. Cancers associated with AIDS include Kaposi's sarcoma, certain lymphomas, and cancer of the rectum and tongue.
A variety of approaches are being developed to treat AIDS infections. These approaches include the development of a means to inhibit the binding of the virus to host cell receptors with agents, such as dextran sulfate, or a soluble form of the CD4 receptor protein (a glycoprotein that specifically binds to the T4 receptors on the surface of certain T cells). Other approaches include the administration of anti-idiotypic antibodies (an antibody to the antibody against CD4), blockage of viral protein synthesis by compounds such as phosphorothioate, and inhibition of protein glycosylation by compounds such as 2-deoxy-D-glucose. These approaches, however, are still in early experimental phases, and have not been approved for clinical treatment.
Antiviral chemotherapy now represents the major approach in preventing and/or treating AIDS. AIDS and ARC chemotherapy has been recently reviewed by Schinazi, Strategies and Targets for Anti-Human Immunodeficiency Virus Type 1 Therapy, "Aids in Children, Adolescents, and Heterosexual Adults: An Interdisciplinary Approach to Prevention", (Elsvier, N.Y., 1988). See also: E. D. Clercq, J. Med. Chem. 29, 1561-1569 (1986); H. Mitsuya, S. Broder, Nature 325, 773-778 (1987); and R. Yarchoan, et al., "AIDS Therapies" The Science of AIDS Scientific American (W. H. Freeman and Co. N.Y. 1989).
A number of nucleoside derivatives have been found to have anti-HIV activity, including 3'-azido-3'-deoxythymidine (AZT), 2',3'-dideoxycytidine (DDC), 2',3'-dideoxyadenosine (DDA), 3'-azido-2',3'-dideoxyuridine (referred to variously as AzdU, AzddU, or CS-87), 2',3'-didehydro-2',3'-dideoxycytidine, 3'-deoxy-2',3'-didehydrothymidine, 3'-azido-5-ethyl-2',3'-dideoxyuridine (AzddEU), 3'-azido-5-methyl-2',3'-dideoxycytidine (AzddMeC), 9-(2,3-dideoxy-2-halo-.beta.-D-arabinofuranosyl)-N.sup.6 -methyladenine (2'-halo-D.sub.2 MeA), and N.sup.6 -methyl-D-glycero-2',3'-dideoxyfuranosyladenosine (D.sub.2 MeA).
3'-Azido-2',3'-dideoxyuridine, AzdU, is a nucleoside analog that inhibits HIV replication in a variety of HIV-infected cells at concentrations close to or below 0.1 .mu.M (U.S. Pat. No. 4,916,122 to Chung K. Chu and Raymond F. Schinazi; Schinazi, R. F., C. K. Chu, M. K. Ahn, and J. P. Sommadossi, J. Cell Biochem. Suppl. 11D, 74 (1987); Chu, C. K., R. F. Schinazi, B. H. Arnold, D. L. Cannon, B. Doboszewski, V. B. Bhadti, and Z. Gu, Biochem. Pharmacol. 37, 3543-3548 (1988); and Chu, C. K., R. F. Schinazi, M. K. Ahn, G. V. Ullas, and Z. P. Gu, J. Med. Chem. 32, 612-617 (1988)). This concentration is at least 200-fold less than that which inhibits colony formation of human bone marrow cells (BMC) (Sommadossi, J. P., Z. Zhou, R. Carlisle, M. Y. Xie, D. A. Wiedner, and M. H. el Kouni, Pharmacol. Ther. (1990), in press). AzdU has been found to inhibit HIV in vivo in clinical trials under the supervision of the U.S. Food and Drug Administration.
AzdU is significantly less toxic than AZT, although it is also slightly less effective at inhibiting the HIV virus than AZT in certain cells. The triphosphorylated form of AzdU, AzdU-TP, competitively inhibits HIV-RT with a K.sub.1 value of 0.0059 .mu.M and inhibits cellular DNA polymerase alpha with a value of 51.5 .mu.M (B. F. H. Eriksson, C. K. Chu, and R. F. Schinazi, Antimicrob. Agents Chemother. 33, 1729-1734 (1989)). These K.sub.i values are similar to those observed with AZT-TP (Cheng, Y. C., et al. (1987)). However, the affinities of AzdU and AZT for thymidine kinase are significantly different, with catalytic efficiency values of 4.6 and 162, respectively. The data suggests that the monophosphorylation step may be of importance in the different behavior of the two drugs in vivo.
It is generally accepted that the active form of nucleosides such as AzdU, AZT, AzddMeC, D.sub.2 MeA, and DDC is the triphosphorylated derivative. Triphosphorylated deoxynucleosides appear to inhibit the replication of HIV by limiting the production of viral DNA by at least two mechanisms: competitive inhibition of reverse transcriptase and chain termination of viral DNA due to the missing 3'-hydroxyl group.
One critical factor in the ultimate therapeutic effectiveness of a nucleoside is how easily the nucleoside can enter the target cells and undergo phosphorylation by cellular enzymes. The efficiency of this process varies considerably among nucleosides. Despite the fact that triphosphorylated nucleosides may be the antivirally active form, they are not clinically useful, without modification, since they cannot pass through the cell membrane.
Not only are nucleosides phosphorylated intracellularly, they are also converted by intracellular enzymes into therapeutically less active metabolites. If the conversion rate to less active compounds is faster than the rate of triphosphorylation of the nucleoside, the pharmaceutical effectiveness of the nucleoside is diminished. For example, it is known that DDA in the triphosphate form is a potent HIV inhibitor in vitro, but in vivo the enzyme adenosine deaminase rapidly converts DDA to the less active DDI (2',3'-dideoxyinosine) before DDA can be phosphorylated. DDI-5'-monophosphate must then be converted to DDA-5'-monophosphate by cellular enzymes to restore the activity of the compound.
The therapeutic effectiveness of a drug is the determining factor in the dosage required for therapy. Nucleosides that pass through the cell membrane with difficulty or which are metabolized into less active or inactive forms in the cell must be administered in higher dosages. Unfortunately, most nucleosides are toxic to healthy uninfected cells at high dosage levels.
Certain 5'-diphosphorylated sugar metabolites of naturally occurring nucleosides play an important biological role in vivo, for example in the synthesis of oligosaccharides, polysaccharides, glycolipids, and glycoproteins, and as components of bacterial cell membranes. Endogenous sugar nucleotides include various derivatives with different sugar moieties including glucose, galactose, and N-acetyl-hexosamines (Datema, R., S. Olofsson, and P. A. Romero Pharamacol. Ther. 33, 221-286 (1987)). Certain nucleoside derivatives have also been found to block the glycosylation of proteins. Most, if not all, known nucleoside glycosylation inhibitors, however, show little selectivity and have low activity against viral infections.
Camarasa, et al., J. Med. Chem. 28, 40 (1985), reported that certain uridine 5'-diphosphoglucose analogues, 5'-O-[[[[(2",3",4",6"-tetra-O-benzyl- and 2",3",4",6"-tetra-O-benzoyl-.alpha.-D-glucopyranosyl)oxyl]carbonyl]amino]s ulfonyl]-2',3'-isopropylideneuridine (P-536), and the corresponding deisopropylidenated derivatives, show in vitro antiviral activity against herpes simplex virus type 1. It has been reported by Alarcon, et al., that P-536 has broad antiviral activity, including activity against adenovirus type 5, vaccinia virus, and poliovirus type 1. Antimicrobial Agents and Chemotherapy 1257, (1988). The compound was demonstrated to inhibit protein glycosylation, if added at a time when late viral proteins were being synthesized, and to inhibit the synthesis of nucleic acids and phosphorylation of nucleosides. Alcina, et al., Antimicrobial Agents and Chemotherapy 1412 (1988), later described that the same compound has activity against the flagellated protozoan Trypanosoma cruzi.
In light of the above, there is an immediate serious need for a new chemotherapeutic agent for the treatment of AIDS that can readily traverse the cell membrane and that is not rapidly metabolized to inactive or toxic metabolites before it can inactivate the virus. In addition, because the progression of AIDS is characterized by increasing susceptibility to opportunistic bacterial, fungal, parasitic, and viral infections, there is a strong need to develop a chemotherapeutic agent that is at the same time effective against at least some of these opportunistic infections.
It is therefore an object of the present invention to provide nucleoside derivatives that can easily pass through a cell membrane in the proper chemical form to perform a desired biological function, or a chemical form that can be rapidly converted in vivo into the active form, without being substantially inactivated.
It is another object of the present invention to provide new antiviral compositions that have low toxicity towards uninfected cells.
It is a still further object of the present invention to provide a method to enhance the cellular levels of nucleosides.
It is another object of this invention to provide nucleoside derivatives that are active against opportunistic infections, in particular, bacterial infections.